1. Field of the Invention
The present invention relates to a method and an apparatus for applying a coating liquid to a cylindrical substrate. In particular, the present invention relates to a method for producing an electrophotographic photoreceptor that is used in an image forming apparatus such as copiers, printers, and facsimiles and produced by applying a coating liquid to a cylindrical conductive support.
2. Description of the Related Art
An electrophotographic photoreceptor used in an image forming apparatus such as copiers, printers, and facsimiles is formed by applying a photosensitive layer to an outer circumferential surface of a hollow cylindrical conductive support. In recent years, as a result of in-depth development in response to demand for high performance, many of the electrophotographic photoreceptors are provided with a photosensitive layer having a laminated structure in which an intermediate layer, a charge-generating layer and a charge-transporting layer, and further a protective layer for improving the durability as its outer layer in some photoreceptors, are applied sequentially.
In this specification, a layer constituted by an intermediate layer, a charge-generating layer, a charge-transporting layer and a protective layer is collectively referred to as a sensitive layer. The intermediate layer and/or the protective layer are provided for the purpose of improving the performance of an electrophotographic photoreceptor, and are not essential, so that a layer consisting of a charge-generating layer and a charge-transporting layer and a layer in which a charge-generating layer and a charge-transporting layer are constituted as one layer are also referred to as a photosensitive layer.
The photosensitive layer provided in the electrophotographic photoreceptor is required to be a thin film and have a uniform thickness. Therefore, new application methods have been under development and examination in order to realize higher performance by applying an even thinner photosensitive layer with uniform thickness and to allow application at low cost.
As a method for applying a photosensitive layer to the outer circumferential surface of a conductive support that is an original pipe for an electrophotographic photoreceptor, spray coating, immersion coating, blade coating and the like are conventionally known. However, conventionally known application methods have problems such as inability of providing uniform coating films or poor production efficiency.
For example, in spraying coating, coating is performed by spraying a coating liquid from a spray nozzle in the form of fine particles, and therefore the appearance after coating is good. However, since a thickness of a layer formed by one coating is small, it is necessary to repeat coating a plurality of times in order to obtain a desired thickness. Furthermore, when a large amount of coating liquid is used for coating, the coating liquid is dropped, and a coating layer with non-uniform thickness is formed. In addition, since the volatile component in the coating liquid is easily vaporized when the coating liquid is sprayed for coating, the viscosity of the coating liquid is increased and orange peel (a phenomenon in which a orange skin-like swell occurs) occurs in the formed coating layer.
In immersion coating, a cylindrical conductive support that is an original pipe for an electrophotographic photoreceptor is immersed with one end thereof held and the axis of the cylinder being perpendicular to the liquid surface of the coating liquid, and then lifted from the coating liquid. Thus, a photosensitive layer is applied onto the surface of the conductive support, and this method is often used to produce an electrophotographic photoreceptor. However, the thickness of the layer (also referred to as “film thickness”) that is applied by immersion coating depends significantly on the lifting speed at which the conductive support is lifted from the coating liquid, the viscosity of the coating liquid, and the speed at which the volatile component contained in the coating liquid evaporates. Therefore, these factors have to be controlled strictly. Furthermore, since the conductive support is lifted from the coating liquid in the vertical direction, the coating liquid flows down along the surface of the conductive support by gravity effect, and the film thickness of the conductive support on the upper side in the lifting direction is smaller than that on the lower side.
In order to solve the problem of non-uniformity of the film thickness, it is necessary to control the lifting speed strictly. However, the control is difficult, and furthermore in order to form a coating film having a uniform thickness, the lifting speed after immersion has to be slow, which is a basic problem. Furthermore, a coating film is formed on the internal portion and the end face of the conductive support, for which coating is not necessary, and therefore it is necessary to peel the coating film formed on the internal portion and the end face. Moreover, since the conductive support is immersed in the coating liquid, it is necessary that a bath for storing a coating liquid contains constantly the coating liquid in an amount sufficient for the total length of the conductive support to be immersed. Thus, since the coating liquid in an amount that exceeds the amount necessary for coating and forming a film is constantly prepared, the use efficiency of the coating liquid becomes poor. Then, in order to improve the use efficiency, instead of preparing a new coating liquid for each occasion of use, a coating liquid that is newly produced is added in a necessary amount to the coating liquid contained in the storage bath that already has been used, and thus the same coating liquid is used many times. However, the viscosity and the characteristics of the coating liquid are changed over time and are changed with a fine difference from those of the newly added coating liquid. Therefore, the application conditions have to be optimized for each coating operation, which decreases the work efficiency.
In blade coating, in view of a conductive support, a blade is positioned near the conductive support and a coating liquid is supplied to the blade. Then, the coating liquid is applied onto the conductive support with the blade, and the blade is withdrawn after one rotation of the conductive support. This method provides high productivity, but when the blade is withdrawn, a part of the coating film applied to the conductive support is protruded by surface tension of the coating liquid and the film thickness becomes non-uniform.
As another method than the above, there is a roll coating method. In this method, a film of a coating liquid with a regulated thickness is formed on an applicator roll, and while a conductive support located near or in contact with the applicator roll and the applicator roll are rotated, the coating liquid is transferred and applied from the applicator roll to the conductive support. However, also in the roll coating method, when detaching the conductive support from the applicator roll after coating, so-called a liquid trailing phenomenon, that is, a phenomenon in which extra coating liquid is attached to the conductive support by the surface tension of the coating liquid tends to occur, and a joint remains in the coating film because of this liquid trailing phenomenon, and the film thickness becomes non-uniform. As a result, defects occur in the images. Herein, “joint” refers to a portion in which the film thickness is non-uniform because extra coating liquid is attached when detaching the conductive support from the applicator roll.
There are conventional techniques for preventing the occurrence of this joint that have been proposed. For example, after a cylindrical substrate is rotated once or more times and coating is ended, the cylindrical substrate is detached from the coating material supplying roll, and it is attempted to achieve leveling (making the film thickness uniform) by allowing the cylindrical substrate to continue to rotate (see Japanese Unexamined Patent Publication JP-A 3-12261 (1991)). However, in the method disclosed in JP-A 3-12261, it is necessary to perform precise control of film thickness, estimating the amount of coating material storage to be leveled in advance, and it is difficult to eliminate the joint completely.
Moreover, there is a method for preventing the occurrence of a joint by decreasing the film thickness of coating material on the applicator roll when detaching between the applicator roll and the cylindrical substrate from each other after application (see Japanese Unexamined Patent Publication JP-A 11-216405 (1999)). Furthermore, there is a method in which after coating, the relationship between the film thickness of the coating material on the applicator roll and the gap formed by the applicator roll and the cylindrical substrate is defined, and the amount of the coating material on the applicator roll is reduced from this state, and the linkage of the coating material between the applicator roll and the cylindrical substrate is cut (see Japanese Unexamined Patent Publication JP-A 2000-325863). However, in the methods disclosed either in JP-A 11-216405 or JP-A 2000-325863, the joint cannot be eliminated to such an extent that defects in the images are suppressed completely. Furthermore, in these methods, it is necessary to control the application conditions strictly at the time of application and detachment, so that high production efficiency cannot be expected.
Furthermore, there is a method of controlling the speed at which the applicator roll and the cylindrical substrate are detached from each other after coating (see Japanese Unexamined Patent Publication JP-A 11-276958 (1999)). However, also in the method disclosed in JP-A 11-276958 as well as in the methods in JP-A 11-216405 or JP-A 2000-325863, the joint cannot be eliminated to such an extent that defects in the images are suppressed completely.
Furthermore, there is a method of forming a rib in a coating film by changing the circumferential speed between the applicator roll and the cylindrical substrate and detaching the applicator roll and the cylindrical substrate from each other in that state (see Japanese Unexamined Patent Publication JP-A 2000-84472). However, in the method disclosed in JP-A 2000-84472, since a rib is formed in the coating film, there are problems described below. That is, when a solvent having a low boiling point is used, the leveling time to eliminate the rib and make the thickness uniform is not sufficient and therefore the coating film swells in waves. When a solvent having a high boiling point is used, the leveling time can be sufficient, but drying takes a long time, which deteriorates the production efficiency significantly. Furthermore, it is necessary to determine strictly the various conditions such as the roll diameter, the circumferential speed, the gap, the coating material viscosity, the surface tension, etc., in order to form a rib, so that it is difficult to determine the application conditions, and the acceptable range for setting of the composition of the coating liquid or apparatus structure is limited. In particular, since the thickness formed as a charge-generating layer and an intermediate layer is small, it is very difficult to set the conditions for forming a rib and even if a rib is formed, it dries in a short time, and a sufficient time for leveling cannot be ensured. As a result, it is difficult to obtain a layer having a uniform film thickness.